1. Field of the Invention
The present invention relates to semiconductor light emitting devices, and particularly to a semiconductor light emitting device in which a current block layer for current confinement is provided on the sides of a light emitting part.
2. Description of the Related Art
As a semiconductor laser having low threshold current Ith, a semiconductor laser having a separated double hetero junction (SDH) structure that can be formed through one time of an epitaxial growth step (hereinafter, referred to as an SDH semiconductor laser) is known from e.g. Japanese Patent No. 2990837.
For this SDH semiconductor laser, initially a projection part extending along the {110}A plane direction is formed on a substrate having the {100} plane as its major surface. Subsequently, through crystal growth over the major surface of this substrate, a light emitting part formed of a multilayer structure composed of compound semiconductor layers is formed on the {100} plane of the projection part (for convenience, referred to as the projection surface). The light emitting part is formed of e.g. a multilayer structure arising from sequential stacking of a first compound semiconductor layer of a first conductivity type, an active layer, and a second compound semiconductor layer of a second conductivity type. A sectional shape obtained when this light emitting part is cut along a virtual plane perpendicular to the extension direction of the projection part is e.g. a triangle, and the side surface (oblique surface) of the light emitting part is the {111}B plane. In general, the {111}B plane is known as a non-growth surface in MOCVD (Metal Organic Chemical Vapor Deposition, referred to also as MOVPE (Metal Organic Vapor Phase Deposition)), except for special crystal growth conditions. Therefore, in the case of the SDH semiconductor laser, after the light emitting part whose side surface is the {111}B plane is formed, “self growth stop” of the crystal growth of the light emitting part is kept even if the MOCVD is continued thereafter.
In the present specification, the crystal planes shown below are represented as the (hkl) plane and the (hk-l) plane, respectively, for convenience.
(hkl)plane
(hk l)plane
In addition, the directions shown below are represented as the [hkl] direction and the [hk-l] direction, respectively, for convenience.
[hkl]direction
[hk l]direction
On the other hand, the {100} plane part as the major surface of the substrate except the projection part (for convenience, referred to as the recess surface) does not involve a non-growth surface. Thus, if the MOCVD is continued, a compound semiconductor layer formed through crystal growth from the recess surface will completely cover the light emitting part in the self growth stop state in time. The compound semiconductor layer formed through crystal growth from the recess surface has, on the second compound semiconductor layer, a structure arising from sequential formation of a layer for adjustment of the current block layer position (hereinafter, referred to simply as the adjustment layer) a current block layer, and a burying layer. Controlling the thickness of the adjustment layer makes it possible to form a structure that permits current injection only to the active layer of the light emitting part through formation of the current block layer at an intermediate phase before the compound semiconductor layer formed through crystal growth from the recess surface covers the light emitting part (in particular, when the upper surface of the compound semiconductor layer is about to reach both the side surfaces of the active layer formed in the light emitting part).
In this manner, for the SDH semiconductor laser, the respective compound semiconductor layers can be formed based on one time of a crystal growth step. In addition, the active layer can be completely surrounded by compound semiconductor layers favorable for light confinement by selecting materials whose energy band gaps are sufficiently wider than that of the active layer, i.e., materials having lower refractive indexes, as the materials used for the compound semiconductor layers that vertically sandwich the active layer in the light emitting part (the first compound semiconductor layer and the second compound semiconductor layer) and the materials used for the current block layer, the burying layer, and the adjustment layer located outside the light emitting part. Due to this feature, the shape of a beam emitted from the semiconductor laser whose light emitting surface is the end surface of the projection part can be brought close to a perfect circle. That is, as the far field pattern (FFP), the following relationship can be achieved.θ//≈θ⊥
Furthermore, depending on e.g. the efficiency of coupling with a lens, it is often needed that the shape of a beam emitted from the semiconductor laser is an ellipse. For such a case, the θ// of the FFP can be set small e.g. by employing a so-called flare-stripe structure, in which the width of the projection part near the ends of the projection part is increased (refer to e.g. Japanese Patent No. 3399018). Moreover, employing the flare-stripe structure can achieve high light output.
For the above-described SDH semiconductor laser, enhancement in the quality of the current block layer (the degree of suppression of current leakage) is a very important technical factor.
The outline of a method for manufacturing a related-art semiconductor laser having a flare-stripe structure will be described below.
[Step-10]
Initially, on the {100} crystal plane, e.g. the (100) crystal plane, of an n-GaAs substrate 10 as its major surface, a light emitting part forming region 11 that has a predetermined width and a substantially-stripe shape and extends along the [011]A direction is formed. The width direction of the light emitting part forming region 11 is parallel to the [0-11]B direction. In this way, the structure shown in FIG. 32A can be obtained. The light emitting part forming region 11 has the oblique surfaces (side surfaces) corresponding to the {111}B plane. The planar shape of the light emitting part forming region 11 is schematically shown in FIG. 32B. The light emitting part forming region 11 has a strip shape in which the width of the center part is smaller than that of both the end parts. In FIG. 32B, the light emitting part forming region 11 is hatched for clearly showing it.
[Step-20]
Subsequently, based on normal MOCVD, specifically, MOCVD with use of an organic metal and a hydrogen compound as the source gas, a buffer layer 12, an n-type first compound semiconductor layer 21, an active layer 23, a p-type second compound semiconductor layer 22 are epitaxially grown over the projection surface and the recess surface. At this time, the oblique surfaces (side surfaces) of the compound semiconductor layers above the projection surface correspond to the {111}B plane. As described above, the {111}B plane is a non-growth surface Therefore, the multilayer structure formed by the buffer layer 12, the first compound semiconductor layer 21, the active layer 23, and the second compound semiconductor layer 22 (so-called double heterostructure) is so formed (stacked) that the double heterostructure in the region above the projection surface is separated from that in the region above the recess surface (i.e., a separated double heterostructure is obtained). By properly selecting the width and thickness of the light emitting part forming region 11 (projection surface) and properly selecting the thicknesses of the buffer layer 12, the first compound semiconductor layer 21, the active layer 23, and the second compound semiconductor layer 22, a multilayer structure of a light emitting part 20 having a triangular sectional shape can be obtained above the center part of the light emitting part forming region 11 (projection surface). On the other hand, at both the end parts of the light emitting part forming region 11, a multilayer structure of the light emitting part 20 having a trapezoidal sectional shape can be obtained at this moment.
[Step-30]
Thereafter, continuously with the formation of the second compound semiconductor layer 22, a layer 30 for adjustment of the current block layer position (hereinafter, referred to simply as the adjustment layer 30), formed of a p-type compound semiconductor layer, is formed across the entire surface based on MOCVD. In this way, the sectional structure shown in FIG. 33 can be obtained at the center part of the light emitting part forming region 11. On the other hand, at both the end parts of the light emitting part forming region 11, the sectional structure shown in FIG. 34 can be obtained at this moment. Furthermore, for example, a current block layer 40 formed of a multilayer structure composed of a p-type compound semiconductor layer and an n-type compound semiconductor layer is formed based on MOCVD. The current block layer 40 is not grown on the {111}B plane. The current block layer 40 is so formed that the end surfaces of the current block layer 40 cover at least the side surfaces of the active layer 23. Such configuration and structure can be achieved by properly selecting the thickness of the adjustment layer 30.
As shown in FIG. 35, at the center part of the light emitting part forming region 11, the current block layer 40 is formed only on the side surfaces of the light emitting part 20. At this moment, as shown in FIG. 36, at both the end parts of the light emitting part forming region 11, in addition to the formation of the current block layer 40 on the side surfaces of the light emitting part 20, the same multilayer structure as that of the current block layer 40 is formed above the top surface ({100} plane) of the multilayer structure of the light emitting part 20 in such a way that the {111}B facet planes (side surfaces) are gradually formed and thus the width of the top surface is gradually decreased. The same multilayer structure as that of the current block layer 40, formed above the top surface of the multilayer structure of the light emitting part 20, will be referred to as a deposited layer 40″, for convenience. Between the deposited layer 40″ and the top surface of the multilayer structure of the light emitting part 20, a compound semiconductor layer 30′ having the same configuration as that of the adjustment layer 30 is formed.
Subsequently, a burying layer 31 and a contact layer (cap layer) 32 are sequentially formed across the entire surface based on MOCVD. At this moment, at both the end parts of the light emitting part forming region 11, the burying layer is formed on the top surface ({100} plane) of the deposited layer 40″ in such a way that the {111}B facet planes (side surfaces) are gradually formed and thus the width of the top surface is gradually decreased. Furthermore, if the width of the top surface is sufficiently large, the same multilayer structure as that of the contact layer (cap layer) 32 is formed. The burying layer on the deposited layer 40″ will be represented as a burying layer 31″. Thereafter, a second electrode 52 is formed based on vacuum evaporation on the contact layer 32 formed as the outermost layer. Furthermore, the substrate 10 is lapped to a proper thickness from the backside thereof, and then a first electrode 51 is formed based on vacuum evaporation (see FIGS. 42 and 43).